The invention relates to an incremental rotary transmitter of the kind comprising a rotatably mounted pulse disc and a relatively stationary scanning grid.
Incremental rotary transmitters have been long known in the art and serve principally for automatic angular measurement. A special field of application is that of the measurement, supervision and control of rpm. Rotary transmitters of the above kind are employed to convert an angular movement of a shaft into digital electrical signals. For this purpose a pulse disc is employed which is coupled to the transmitter shaft and carries a pattern of stripes uniformly distributed about its periphery. The pulse disc is illuminated from one side, and at the side of the pulse disc remote from the light there is arranged a fixed scanning grid with associated photoelectric elements. These latter convert into electrical signals the variations in luminosity resulting from the rotation of the pulse disc.
The determining factor for the digital resolution of an angular variation is the number of stripes per unit angle carried upon the pulse disc of the transmitter, or the so-called stripe number or density. For practical applications in which a particularly high level of resolution is required, pulse discs are employed having a very high stripe density (up to 20,000 stripes around the disc). Such a high stripe density means that the stripes and the intervening gaps are very narrow, so that small eccentricity errors of the pulse disc, due to the usual manufacturing tolerances, will of themselves be the cause of measurement errors. This error source becomes particularly important if the pulse disc is mounted upon the end of a comparatively narrow shaft in which flexing oscillations can be set up, or upon a shaft which does not run true. In this case there will appear lateral fluctuations about the centre point with respect to the scanning grid, which is fixed at a distance of a fraction of a millimeter from the pulse disc, which fluctuations will exacerbate the above mentioned errors.
A device is already known (DBP 20 39 893) wherein a non-rotatably arranged member is mounted upon the rotatable part of the device. However, in that case the subject matter relates to a rotating electromagnetic transducer, of which the instantaneous angular position is immaterial.